Another big difference from the first Earth Day: Many will mark the day digitally in 2011—thanks in no small part to today's Google Doodle, which publicizes Earth Day with an impossible animated Eden where pandas, penguins, and lions live in peace and an iceberg is just a pounce away from an African savanna.

And just this past Tuesday, the Washington, D.C.-based Earth Day Network—founded by the organizers of the first Earth Day—launched a new Facebook Web app to promote their Billion Acts of Green campaign. The project aims to generate a billion pledges of environmental service and advocacy before the Rio+20 Earth summit in Brazil from June 4 to 6, 2012.

"This is really about moving people beyond environmental education and getting everybody to commit to at least one thing—as evidence, we hope, to the world governments that one in seven people on the planet has taken an action," said Kathleen Rogers, president of Earth Day Network.

Every day, the saying goes, is Earth Day. But it's popularly celebrated on April 22. Why?

One persistent rumor holds that April 22 was chosen because it's the birthday of Vladimir Lenin, the founder of the Soviet Union.

"Lenin's goal was to destroy private property and this goal is obviously shared by environmentalists," according to a 2004 Capitalism Magazine article perpetuating the theory.

Likewise, shortly after the first Earth Day, a Daughters of the American Revolution member lamented to Time magazine, "Subversive elements plan to make American children live in an environment that is good for them."

The Earth Day Network's Rogers, though, scoffed at the rumored communist connection.

April 22, 1970, was chosen for the first Earth Day in part because it fell on a spring Wednesday—ideal for encouraging a large turnout, especially of college students, for an environmental event held across the United States, Rogers said.

"It worked out perfectly, because everybody was at work and they all left," she said.

More than a billion people in 180 countries around the world now celebrate Earth Day every year, Rogers said.

Roots of Earth Day

Earth Day's history is rooted in the 1960s, when the environment was in visible ruins and people were angry, according to Rogers.

"It wasn't uncommon in some cities during rush hour to be standing on a street corner and not be able to see across the street" because of pollution, she said.

Despite the anger, green issues were absent from the U.S. political agenda. This frustrated U.S. Senator Gaylord Nelson of Wisconsin, whose campaigns for the environment through much of the 1960s had fallen flat.

In 1969 Nelson hit on the idea of an environmental protest modeled after anti-Vietnam War demonstrations called teach-ins.

"It took off like gangbusters. Telegrams, letters, and telephone inquiries poured in from all across the country," Nelson recounted in an essay shortly before he died in July 2005 at 89.

"The American people finally had a forum to express its concern about what was happening to the land, rivers, lakes, and air—and they did so with spectacular exuberance." (See pictures of quirky Earth Day stunts.)

Nelson recruited activist Denis Hayes to organize the April 22, 1970, Earth Day teach-in, which today is sometimes credited for launching the modern environmental movement.

By the end of 1970, the U.S. Environmental Protection Agency had been born, and efforts to improve air and water quality were gaining political traction.

She noted that, since the first Earth Day, environmentalism has moved from a fringe issue to a mainstream concern. "As many as 80 percent of Americans describe themselves as environmentalists," Cassara said. (Learn how you can help this Earth Day.)

Environmental issues today, however, are less immediate than dirty air, toxic water, and a hole in the ozone layer, she added.

For example, the impacts of global warming are largely abstract and difficult to explain "without coming off as a doomsday prognosticator," Cassara said.

As the world becomes "more industrialized and our supply chains become less transparent, it can be more difficult to understand the environmental consequences of our actions," she said.

Earth Day Has Lasting Impact

Earth Day Network is pushing the Earth Day movement from single-day actions—such as park cleanups and tree-planting parties—to long-term commitments.

"Planting a tree, morally and poetically, requires taking care of it for a really long time, not just sticking it in the ground," Earth Day Network's Rogers said.

To help make the transition, the organization has engaged a hundred thousand schools around the world, integrating environmental themes into the year-round curriculum.

"They announce the results on Earth Day, so Earth Day becomes a moment in time," Rogers said.

The World Resources Institute's Cassara added that her organization uses Earth Day to convene with leaders in the movement and assess progress in their campaigns.

Now that it's so widely known, Earth Day "doesn't raise awareness among the general public in the same way that it used to. But it still provides a benchmark for reflection among those of us in the environmental community," she said.

Don't expect to find Keith Lockitch, a fellow with the Ayn Rand Institute in Irvine, California, at any Earth Day rallies.

Named for late novelist and philosopher Ayn Rand, the institute promotes the "principles of reason, rational self-interest, individual rights, and laissez-faire capitalism to the widest possible audience," according to its Web site.

Much as libertarian groups oppose late March's Earth Hour, the Ayn Rand Institute opposes Earth Day as a celebration of the anti-capitalist environmental movement, Lockitch said.

"Whenever there is a conflict between protecting nature from any kind of human activity versus pursuing some human value, environmentalists will consistently side with nature," he said.

For example, he said, environmentalists protest the construction of hydroelectric dams because they might affect species' habitat—even though industrial society needs electricity to survive.

Instead of celebrating the environmental movement, Lockitch said, "we should have a Be Proud of Being Human Day … not a Feel Guilty for Trampling on Mother Earth Day."